1. Field of the Invention
The present invention particularly relates to a dynamic microphone in which a model provided with an output switch for a sound signal from a microphone unit can be easily diverted to a switchless model.
2. Background of the Related Art
A dynamic microphone is also called an “electrodynamic microphone”, since a voice coil attached to a diaphragm is disposed in a magnetic gap formed in a magnetic circuit, and a sound signal current is generated in the voice coil by vibration of a diaphragm.
In order to obtain a good frequency characteristic in a low bandwidth, this dynamic microphone is arranged such that an air room is provided at the back of a microphone unit and the back of the diaphragm is communicated with the above-mentioned air room so as to obtain a non-directional component.
Since it is desirable that the volume of the air room formed at the rear of the above-mentioned microphone unit is large, the whole inside of a microphone case may be used as an air room.
FIG. 8A shows an example in section. A microphone unit 1 provided with a voice coil and a diaphragm to which a magnetic circuit is attached is fitted in a microphone case 6 through a holder 4 and a shock mount member 5 formed of an elastic material (for example rubber) in the shape of a cylinder.
Further, the back of the diaphragm in the microphone unit 1 is communicated with the above-mentioned microphone case 6, and it is arranged that the whole interior space of the microphone case 6 is used as a back air room 6a. 
It should be noted that reference numeral 8 indicates a seal member arranged in the vicinity of the output connector at the rear end of the microphone case 6.
Further, the above-mentioned microphone may be of a model provided with a switch for turning on or off (ON/OFF control) the sound signal output from the microphone unit.
FIG. 8B shows an example thereof in section, in which an opening 6b is formed at a side wall of the microphone case 6 and a microphone switch 9 is attached to this opening 6b. 
In this case, since an air leak may occur between the above-mentioned opening 6b provided for the microphone case 6 and the microphone switch 9, the microphone unit 1 is mounted in the microphone case 6 through the cup-shaped shock mount member 5 whose bottom is sealed.
Therefore, in the example shown in FIG. 8B, since the inside of the cup-shaped shock mount member 5 is a sealed back air room 5a and the above-mentioned microphone switch 9 is provided, there are restrictions on the volume of air room 5a formed in the shock mount member 5.
Then, it is possible to employ a structure shown in FIG. 8C in which the microphone switch 9 is mounted at the side wall of the microphone case 6, while employing the structure in which the microphone unit 1 is mounted in the microphone case 6 using the shock mount member 5 formed in the shape of a cylinder as shown in FIG. 8A.
According to the structure shown in FIG. 8C above, although the whole inside of the microphone case 6 can be used as the back air room 6a of the microphone unit 1, some treatments are needed, such as filling a gap with for example silicon resin etc., or sealing the gap by a component like rubber packing member, in order to prevent the air leak between the opening 6b formed at the side wall of the microphone case 6 and the microphone switches 9.
As described above, in the model provided with the output switch 9 of the sound signal from the above-mentioned microphone unit 1, it is necessary to consider the arranged position of the above-mentioned switch 9 in order to enlarge the back air room of the microphone unit 1. As disclosed by for example, Japanese Patent Application Publication No. H6-225376 and Japanese Patent Application Publication No. 2009-218685, it is possible to arrange the above-mentioned switch at the output connector portion at the rear end of the microphone case 6.
Incidentally, depending on the scene where the microphone is used, different decision is made as to whether the output switch for the sound signal is required for the microphone side.
For example, when using a microphone on a stage etc., a skilled adjustment personnel performs input operation of the microphone by means of a control board in order to prevent using the microphone with the above-mentioned switch being turned off accidentally, and in order to avoid generation of electric crashing noises caused by switching operation etc. In this case, the switchless model is required for the microphone.
Further, when using it for a lecture meeting, a speech, etc., a model with a switch is required since a user himself or herself performs on/off control by hand as needed.
For this reason, also in the microphone of substantially the same specification and performance, both types, a model with a switch and a switchless model, must be produced commercially in some cases, and it is necessary to design, manufacture, manage, etc. each model, leading to a problem that manufacture costs etc. increase for each model.